1. Field of the Invention
The present disclosure relates to an image forming apparatus, such as a copying machine and a printer, having a function of forming an image on a recording material such as a sheet.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus, due to advantages such as low ozone and low power consumption, an apparatus using a method employing a contact charging device has been put into practical use. The contact charging device is a charging member which abuts on a photosensitive member (image carrier) and to which voltage is applied to charge the photosensitive member. Particularly, a roller charging apparatus using a charging roller as the charging member has been widely used from the viewpoint of charging stability. Recently, from the viewpoint of low cost and space saving, a charging method (hereinafter, referred to as a DC charging method) in which only DC voltage is applied to the charging roller is used (Japanese Patent Application Laid-Open No. 8-171260).
However, in the DC charging method, it is difficult to uniform a potential difference (hereinafter, referred to as a transfer memory) of the photosensitive member after transferring, and the transfer memory may appear as an image.
The transfer memory is a phenomenon in which flow amounts of transfer current to the photosensitive member are different between a portion carrying the toner and a portion without the toner and thus a difference in potential appears on the photosensitive member after transferring, and the potential is not sufficiently made uniform during the charging process. As a consequence, the difference in potential appears on the image occurs. For this reason, conventionally, to uniform the surface potential of the photosensitive member after transferring, light neutralization is carried out by a memory removing unit. However, installation of the memory removing unit causes increase in size of an apparatus and increase in cost.
Accordingly, so-called background exposure is known as a method for suppressing the transfer memory without separately providing the memory removing unit (Japanese Patent Application Laid-Open No. 2008-8991). By the background exposure, while the photosensitive member is charged at a predetermined potential during the charging process, a portion carrying a toner image is exposed by an exposure unit and simultaneously, a blank portion in which the toner image is not formed is also exposed to a small light amount.
However, the photosensitive member is gradually subjected to optical-fatigue by the exposure. As a result, in a system of exposing the surface of the photosensitive member at all times like the background exposure, the decrease in photosensitivity due to the optical fatigue of the photosensitive member needs to be considered.
Recently, a long lifespan and high image quality of products have advanced, and users have become diversified. Accordingly, even in the photosensitive member, it is required that product performance remains stable for a long time. In order to achieve the long lifespan, it is important that the optical fatigue of the photosensitive member is reduced as much as possible and the decrease in sensitivity is suppressed.